1. Field of the Invention
This invention relates to the control of the contrast on liquid crystal display (LCD) panels, and more particularly to the control of a contrast bias voltage used to regulate the contrast on the LCD panel.
2. Description of the Related Art
While the portability of laptop computers is advantageous in many instances, they are often used connected to external VGA CRT monitors, which run in various modes. The raster-scan monitors presently used are volatile, and thus an entire frame must be refreshed at a repetition rate above the flicker level of the human eye. In the United States, the standard for the minimum repetition rate is set at 60 Hz per frame to avoid flicker. The frame consists of horizontal scan lines that are scanned one line at a time, generally from the top of the screen down to the bottom. VGA modes utilize two possible frame sizes: 449 scan lines per frame for lower resolution modes or 525 scan lines per frame for higher resolution modes. In addition, the laptop computer runs its internal LCD panel using 480 horizontal scan lines per frame. As a result, there are three possible refresh modes that the video controller must be able to handle: the 449-line mode, the panel-only 480-line mode, and the 525-line mode. In laptop computers, the panel-only 480-line mode is the optimum mode; because the LCD panel is typically organized into two vertical halves, each half will have 240 lines in the 480-line refresh mode. No additional lines are needed to handle the vertical retrace period present on raster scan monitors. However, current laptop computers can display simultaneously on the internal LCD panel and the external monitor. Because of the nature of the external monitor, the internal LCD panel must be run at the same scan rates as the external monitor, even though a maximum of 480 lines can be displayed. For the two monitor refresh modes, the number of lines in each half of the LCD panel is then organized unevenly. In the 449-line mode, the top half contains 224 lines, and the bottom half contains 225 lines. In this mode there are blank areas at the top and the bottom of the LCD panel. In the 525-line mode, the top half contains 240 lines, and the bottom half is provided 285 lines but displays only 240 lines.
Thus, in the 449-line mode, the vertical scan rate decreases from the LCD 480-line scan rate. Furthermore, 49 of the 449 horizontal scan lines are dedicated to the vertical refresh period in the case of the CRT monitor. Therefore, there are only 400 active lines scanned. As a result, 40 lines in the top half and 40 lines in the bottom half of the LCD panel are left blank. In the case of the 525-line mode, 45 horizontal lines are dedicated to the vertical retrace period, leaving 480 active lines. In this mode, the scan rate increases to account for the extra lines, but those extra lines are dropped off at the end of the 480-line frame.
In VGA, the total horizontal scan period stays constant as the number of horizontal scan lines changes. This means that the vertical scan rate changes as the number of scan lines changes. The rate is lower when the number of lines is greater. The change in the vertical rate along with the change in the number of lines means that the effective duty cycle of each scan line changes. The percentage of time that a single line is being driven on the LCD panel is lower as the number of lines increases. This time difference causes the contrast perceived by the user to change as the output modes are changed. The change is noticeable in the common cases and so is a source of annoyance.
Typically, this problem is remedied by adjusting the manual contrast controls, but this is time consuming and requires activity on the part of the user increasing the annoyance. The contrast variation problem can not simply be fixed in software by modifying the video driver in the video BIOS. It would seem that the driver could simply change the contrast setting on controllers having software selectable contrast. However, because many programs circumvent the video driver in the video BIOS, a video driver solution is not a complete solution. Therefore it is desirable to have a video controller which automatically adjusts contrast settings as video modes are changed.